PERSPECTIVE https://doi.org/10.1038/s41558-018-0326-3 Antarctic surface hydrology and impacts on ice-sheet mass balance Robin E. Bell 1*, Alison F. Banwell 2,3, Luke D. Trusel 4 and Jonathan Kingslake 1,5 Melting is pervasive along the ice surrounding Antarctica. On the surface of the grounded ice sheet and floating ice shelves, extensive networks of lakes, streams and rivers both store and transport water. As melting increases with a warming climate, the surface hydrology of Antarctica in some regions could resemble Greenland’s present-day ablation and percolation zones. Drawing on observations of widespread surface water in Antarctica and decades of study in Greenland, we consider three modes by which meltwater could impact Antarctic mass balance: increased runoff, meltwater injection to the bed and meltwa- ter-induced ice-shelf fracture — all of which may contribute to future ice-sheet mass loss from Antarctica. urface meltwater in Antarctica is more extensive than previ- Through-ice fractures are interpreted as evidence of water having ously thought and its role in projections of future mass loss drained through ice shelves (Figs 1a and 2)15. Similar to terrestrial Sare becoming increasingly important. As accurately projecting hydrologic systems, these components of the Antarctic hydrologic future sea-level rise is essential for coastal communities around the system store, transport and export water. In contrast to terrestrial globe, understanding how surface melt may either trigger or buf- hydrology, on ice sheets and glaciers water can refreeze with conse- fer rapid changes in ice flow into the ocean is critical. We provide quences for the temperature of the surrounding ice16–18, firn or snow. an overview of the current understanding of the major components Storage occurs in lakes, crevasses, in buried lakes and possibly in of the Antarctic surface hydrology system and the distribution of firn aquifers. Transport and export are less persistent and more dif- melt. Using the framework of surface hydrology in Greenland, we ficult to observe than lakes14,19,20. Antarctic surface and subsurface consider the different ways in which surface hydrology can impact hydrological systems have been studied using satellite and airborne ice-sheet mass balance. Looking to the future, we discuss how the imagery1,8,11–14, although field-based observations are limited8,21–23. hydrologic systems will evolve in Antarctica as well as their impact on future changes in ice-sheet mass balance. Finally, we highlight Surface storage of meltwater. Meltwater is stored in surface lakes on knowledge gaps that limit our understanding of the impact of both grounded and floating ice. On grounded ice, lakes develop in increased surface meltwater on future sea level rise. areas with local-scale melt enhancement and relatively low accumu- lation rates; areas that are often close to rock outcrops and blue ice Current distribution of meltwater (for example, Shackleton Glacier; Fig. 1j)12,14. Similar to Greenland20, Meltwater on the surface of Antarctica was observed by early explor- on grounded Antarctic ice, lakes form in persistent surface depres- ers, who noted the noise of running water and water seeping into sions. The formation of surface lakes in the same location each their tents1. Today the surface melt distribution in Antarctica (Fig. 1) year over decades is evidence of control by the interplay between is determined using satellite observations2–5 and reanalysis-forced bedrock topography and ice flow24. On the floating ice shelves sur- regional climate modeling6. The surface meltwater production esti- rounding Antarctica8,14,19,20,25, water collects in surface depressions mates derived from these two methods correspond well with in situ that move with ice flow. These surface depressions in the ice shelf observations7. At present, the most intense melt is observed across are controlled by basal crevassing26, grounding zone flow-stripe ice shelves (Fig. 1), particularly along the Antarctic Peninsula, development27, suture-zone depressions1 and basal channels pro- including the Larsen C, Wilkins and George VI ice shelves, as well duced by ocean melting28. Water will fill a depression if the ice sur- as the relatively low-latitude East Antarctic ice shelves, including the face or near surface is impermeable. Impermeable surfaces are often West and Shackleton ice shelves. More localized, but intense, melt associated with high melt and low snow accumulation rates29. Once occurs on other East Antarctic ice shelves, including the Amery and water collects in an ice shelf depression, the basin will deepen due to Roi Baudouin ice shelves7,8, where extensive surface hydrological both enhanced lake-bottom ablation (due to the lower albedo of the networks develop. The two largest ice shelves, Ross and Ronne- water compared to the surrounding ice/snow)30,31, and the flexural Filchner, experience only minor surface melting. The upper eleva- response of the floating ice to the water load32,33. The largest supra- tion limit of surface melting today is generally ~1,400 m during glacial lake (~80 km long) is on the Amery Ice Shelf (Fig. 2e)13,14,34. spatially extensive, but low-magnitude, West Antarctic melt epi- On both grounded and floating ice, surface fractures (crevasses) sodes7,9, compared to a 3,200 m elevation limit in Greenland during can accumulate water26, serving as another storage site for meltwa- the anomalous10 2012 melt events. ter and a mechanism by which water directly impacts ice dynam- Liquid water on the Antarctic Ice Sheet and the floating ice ics. Water-filled fractures may propagate vertically when sufficient shelves that buttress upstream grounded ice (Fig. 1) is found in water is available, creating through-ice fractures on Antarctica’s ice supraglacial lakes, subsurface lakes, surface streams and rivers1,8,11–14. shelves32,35,36 and Greenland’s floating tongues37. Fractures beneath 1Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA. 2Scott Polar Research Institute, University of Cambridge, Cambridge, UK. 3Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA. 4Department of Geology, Rowan University, Glassboro, NJ, USA. 5Department of Earth and Environmental Sciences, Columbia University, Palisades, NY, USA. *e-mail: [email protected] 1044 NatURE CLIMate CHANGE | VOL 8 | DECEMBER 2018 | 1044–1052 | www.nature.com/natureclimatechange NATURE CLIMATE CHANGE PERSPECTIVE a b c Meltponds and dolines Foehn-enhanced melt ponds Subsurface lake f 20 km 20 km George VI Ice Shelf Larsen C Ice Shelf Western Roi Baudouin Ice Shelf 10 January 2003 7 February 2016 January/February 2016 j d Southernmost large-scale drainage Ice shelf stream and moulin c d Antarctic Peninsula Dronning Maud Land b a e Shackleton Glacier Eastern Roi Baudouin Ice Shelf 10 January 2010 January/February 2016 e i West Antarctica East Antarctica High-elevation stream Largest supraglacial lake i f j h g Surface melt (mm w.e. yr–1) 80 km 0 100 200 300 400 500 Law Glacier (~1,830 m a.s.l.) Amery Ice Shelf 14 January 2017 21 February 1988 h g f Drainage across grounding line Terminal waterfall Ring and radial fractures, streams and ponds 10 km Darwin Glacier/Ross Ice Shelf Nansen Ice Shelf Shackleton Ice Shelf 31 December 2001 12 January 2014 10 February 1947 Fig. 1 | Examples of major components of surface hydrological systems located on a present-day Antarctic surface melt map. The central map shows 2000–2009 Antarctica surface melt from QuikSCAT satellite observation 7; the locations of the images in a–j are indicated. a, Meltwater lakes and dolines (arrows), b, Foehn wind-enhanced meltwater ponding. c, Buried lake. d, Moulin draining surface stream. e, Elongate supraglacial lake. f, Fractures around a drained lake. Scale unknown. g, Persistent waterfall draining water. h, Supraglacial streams transporting water across grounding line of the Darwin Glacier onto the Ross Ice Shelf. i, High-elevation (1,830 m) meltwater stream. j, Meltwater stream crossing the grounding line. Images reproduced from: US Geological Survey (a,b,e,h); ref. 8, Springer Nature Limited (c); Sanne Bosteels (d); USGS/EROS and the Polar Geospatial Center (f); Won Sang Lee (g); Mike Kaplan (i); John Stone (j). lakes on Greenland’s grounded ice drain meltwater to the ice-sheet line lakes refreeze, they form massive ice layers29,42. Over succes- bed by hydrofracturing38,39. There is no direct evidence of hydro- sive melt seasons, frozen surface lakes (now stacked ice lenses) may fracture beneath lakes on grounded Antarctic ice. accumulate in dense and thick ice horizons29. On the Larsen C Ice Shelf, a massive ice facies > 40 m thick, extending 16 km horizon- Englacial storage of meltwater. Antarctic surface meltwater tally, was interpreted as a stack of frozen lakes42. Temperature pro- is stored englacially when surface lakes freeze over and are bur- files through this refrozen ice are substantially warmer due to the ied by snowfall8,40. In Antarctica, buried lakes tend to form on ice release of latent heat as the lakes froze, similar to the cryohydro- shelves close to the grounding line8. Since at least 1947 on the Roi logic warming described across Greenland17. Baudouin Ice Shelf, meltwater produced in areas of blue ice above In Greenland, perennial firn aquifers store water in environ- and below the grounding line fills surface lakes. These lakes are ments similar to where buried supraglacial lakes form43. Water in buried as the ice moves towards the calving front14. Radar satel- these firn aquifers is stored in a porous matrix of ice crystals. No lites, such as C-Band Sentinel-1 A and B, are capable of penetrating Antarctic firn aquifer has been sampled yet, but beneath massive metres though dry snow, highlighting the promise of tracing buried ice facies on Larsen C, a second ~45-m-thick ice unit has been lakes and other subsurface liquid water41.